There are several things at play and it's difficult to sort them all out. I don't use multiple contexts if I can possibly avoid it (this is why -- are you sure you need another context?
). If the dev context is just for development, it's easier to create a local version of the site on your computer with XAMPP or something similar, and do development work there, imo. Otherwise, when things don't work in the dev Context, you won't know whether it's a problem with the Context itself, or with what you're trying to do.
I'm guessing at some of this, so maybe someone more familiar with multi-context sites can chime in.
First, you need to
create Context settings for the dev context -- things like base_url, site_url, and base_path. That may keep TinyMCE happy -- I'm not sure where TinyMCE gets its base path and base URL settings from.
Second, make sure your base href tag has the exclamation point, so it won't be using cached versions of the site_url:
<base href="[[!++site_url"]] />
Third, I think you'll need to create new Media Sources for the dev context (this will only effect TVs). Then, edit the Media Sources tab of each TV to add the new dev Media Source so it will use the right source in each context. If the TVs will be shared between contexts, I think you may have to use full absolute paths and URLs in the Media Source's properties.
Finally, if users will be logging in and need to access both contexts, you need to add the &contexts property to the Login snippet tag and list both contexts there:
[[!Login? &contexts=`web,dev`]]
